The present invention relates to optical movement information detectors, movement information detection systems, electronic equipment and encoders. More specifically the present invention relates to an optical movement information detector that is used as a Doppler velocimeter for detecting the velocity of an object to be measured by applying laser light to the moving object to be measured and receiving scattered light from the object to be measured including the frequency shift of light corresponding to the movement velocity of the object to be measured as well as a movement information detection system, electronic equipment and an encoder employing the detector. The present invention relates, in particular, to a signal processing technology for detecting a velocity with high accuracy and a signal processing technology for expanding the detectable velocity range, also to a signal processing technology usable for a small-sized Doppler velocimeter and further to electronic equipment with the built-in optical movement information detector that can also be used as a displacement information detector for calculating displacement information on the basis of the velocity information of the object to be measured and time information.
In general, when a light source and an observer are moving relative to each other, light suffers a change in frequency due to the Doppler effect. A laser Doppler velocimeter (hereinafter referred to as LDV) utilizes this Doppler effect and applies laser light to a moving object to be measured to measure the Doppler frequency shift of scattered light and detects the movement velocity of the object to be measured. This LDV has been made public by Yeh and Cummins in 1964 (Appl. Phys. Lett. 4–10(1964)176) and is currently generally well known and put to practical use.
FIG. 11 shows an optical system diagram of a conventional typical differential LDV (refer to, for example, JP 03-235060 A). FIG. 11 shows a semiconductor laser (hereinafter, referred to as LD (Laser Diode)) 101, a photodetector (hereinafter, referred to as PD (Photo Diode)) 102, a diffraction grating 103, a collimator lens (hereinafter, CL) 104, mirrors 105, a condenser lens (hereinafter, referred to as OL) 106, a first light flux 107 and a second light flux 108 of positive and negative first-order lights diffracted by the diffraction grating 103. In this optical system, the laser light emitted from the LD 101 is converted into a parallel light flux by the CL 104 and split into the positive and negative first-order diffracted lights at a diffraction angle θ by the diffraction grating 103 to become the first light flux 107 and the second light flux 108. The light fluxes are reflected on the mirrors 105 and thereafter superposed on the object to be measured at an incident angle θ. The light fluxes scattered by the object 114 to be measured undergo a Doppler frequency shift (±fd) and are different from the oscillation frequency (f0) of the LD 101, and therefore, beat between the interference waves is generated. This is called a beat signal. By subjecting the beat frequency of the beat signal to heterodyne detection by the photodetector 102, the movement velocity of the object to be measured can be obtained. A detailed description is provided below.
Assuming now that the rightward direction is the forward direction as shown in FIG. 11, then the first light flux 107 and the second light flux 108 undergo Doppler frequency shifts of −fd and +fd, respectively, so that the apparent frequency of the first light flux 107 becomes (f0−fd) and the apparent frequency of the second light flux 108 becomes (f0+fd). It is to be noted that f0 is the oscillation frequency of the LD 101. At this time, an electric field of the laser light emitted from the LD 101 can be expressed by:E0·cos(2πf0t)and therefore, the first light flux 107 and the second light flux 108 can be expressed by:First Light Flux: IA=EA·cos {2π(f0−fd)t+φA}  Equation (1):Second Light Flux: IB=EB·cos {2π(f0+fd)t+φB}  Equation (2):It is to be noted that E0, EA and EB represent the amplitudes of the respective lights, φA and φB represent the phases of the respective lights. The frequency of light is generally 100 THz (1014 Hz), and therefore, the frequency information of Equation (1) and Equation (2) cannot directly be measured. Accordingly, the heterodyne detection is generally used as described above, and the expression f0>>fd holds. Therefore, the interference wave of Equation (1) and Equation (2) can be expressed by:
                              Equation          ⁢                                          ⁢                      (            3            )                    ⁢                      :                          ⁢                                  ⁢                              〈                                                                                                I                    A                                    +                                      I                    B                                                                              2                        〉                    =                                                                      E                  A                  2                                +                                  E                  B                  2                                            2                        +                                                            E                  A                                ·                                  E                  B                                ·                cos                            ⁢                              {                                                      2                    ⁢                                          π                      ⁡                                              (                                                  2                          ⁢                                                      f                            d                                                                          )                                                              ⁢                    t                                    -                                      (                                                                  ϕ                        A                                            -                                              ϕ                        B                                                              )                                                  }                                                                                    It is to be noted that the symbol “<>” on the left side of Equation (3) represents a time average. Therefore, the frequency of the interference wave can be measured by the PD 102.
FIG. 12 shows a view when two light fluxes are made incident at arbitrary angles (α, β) and scattered light is received at an arbitrary angle (γ) when the object 114 to be measured is moving at a velocity V. The amount of frequency shift due to the Doppler effect is obtained strictly by using the Lorentz transformation according to the theory of relativity. When the movement velocity V of the object 114 to be measured is sufficiently smaller than the light velocity c, the frequency shift can be obtained through approximation as follows. Relative velocities VA1 and VB1 of the moving object from a light source A and a light source B can be expressed byVA1=c−V sin αVB1=c+V sin β  Equations (4):Apparent frequencies fA1 and fB1 of the lights viewed from the moving object (object 114 to be measured) are expressed by Equations (5):
            f      A1        =                            V          A1                λ            =                        1          λ                ·                  (                      c            -                          V              ⁢                                                          ⁢              sin              ⁢                                                          ⁢              α                                )                                f      B1        =                            V          B1                λ            =                        1          λ                ·                  (                      c            +                          V              ⁢                                                          ⁢              sin              ⁢                                                          ⁢              β                                )                    Relative velocities VA2 and VB2 of the scattered (reflected) lights with respect to the moving object are expressed byVA2=c−V sin γVB2=c−V sin γ  Equations (6):Therefore, the frequencies fA2 and fB2 of the lights viewed from the observation point can be expressed by
                              Equations          ⁢                                          ⁢                      (            7            )                    ⁢                      :                          ⁢                                  ⁢                              f            A2                    =                                                    c                                  V                  A2                                            ·                              f                A1                                      =                                          c                λ                            ·                                                1                  -                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    α                                                                    1                  -                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    γ                                                                                      ⁢                                  ⁢                              f            B2                    =                                                    c                                  V                  B2                                            ·                              f                B1                                      =                                          c                λ                            ·                                                1                  +                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    β                                                                    1                  -                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    γ                                                                                                                      A difference between the frequency in Equations (7) and the frequency of the incident light becomes the Doppler frequency shift amount: fd. The beat frequency of the two light fluxes measured at the observation point becomes expressed by
      Equation    ⁢                  ⁢          (      8      )        ⁢          :                                                2            ⁢                          f              d                                =                                                                                    f                  B2                                +                                  f                  A2                                                                    =                                          V                λ                            ·                              (                                                      sin                    ⁢                                                                                  ⁢                    α                                    +                                      sin                    ⁢                                                                                  ⁢                    β                                                  )                                                                                              according to c>>V, and it can be understood that this does not depend on the position (angle: γ) of the observation point. In FIG. 11,α=β=θand therefore, in the general LDV optical system of FIG. 11, according to Equation (8),
      Equation    ⁢                  ⁢          (      9      )        ⁢          :                                                2            ⁢                          f              d                                =                                                                      2                  ⁢                  V                                λ                            ·              sin                        ⁢                                                  ⁢            θ                                                                    holds. Therefore, by measuring the frequency 2fd expressed by Equation (3) and carrying out calculation using Equation (9), the movement velocity V of the object can be obtained.
Moreover, Equation (9) can be geometrically considered as follows. FIG. 13 is an enlarged view of the region where two light fluxes of FIG. 11 overlap each other again. The two light fluxes are made incident at the incident angle θ and cross each other, and the dashed lines in the figure indicate part of equal wave fronts of the light fluxes. An interval between the dashed lines becomes the light wavelength λ. Moreover, the vertical thick lines indicate the bright portions of the interference fringes, and assuming that an interval of the bright portions is Δ, then Δ is expressed by
      Equation    ⁢                  ⁢          (      10      )        ⁢          :                                    Δ          =                      λ                          2              ⁢              sin              ⁢                                                          ⁢              θ                                                                                As shown in FIG. 13, when the object (indicated by the black dot •) perpendicularly passes through the interference fringes at the velocity V, a frequency f thereof becomes expressed by
                              Equation          ⁢                                          ⁢                      (            11            )                    ⁢                      :                          ⁢                                  ⁢                  f          =                                    V              Δ                        =                                                                                                      2                      ⁢                      V                                        λ                                    ·                  sin                                ⁢                                                                  ⁢                θ                            =                              2                ⁢                                  f                  d                                                                                                    which becomes equal to Equation (9). This way of thinking is called the interference fringe model.
In the LDV described above, a signal obtained through photoelectric conversion by the photodetector is the sum of the DC (Direct Current) component and the AC (Alternating Current) component as expressed by Equation (3). In this case, as expressed by Equation (9) and Equation (11), the LDV takes advantage of the fact that the frequency of the signal has a relation proportional to the movement velocity of the object to be measured, and the parameter to be detected is the frequency. Although the ideal LDV signal oscillates with an amplitude=EAEB around a constant DC level=(EA2+EB2)/2 as expressed by Equation (3), actually the DC signal level is disadvantageously excessively larger than the amplitude of the AC signal due to the coherence of the light source used, the beam spot overlap deviation, variations in the quantity of light of both the light fluxes, the incident angle dependence of the surface reflectance of the object to be measured and so on. If the measurement is carried out directly by the photodetector, the AC signal is disadvantageously buried in the large DC noises and becomes a signal of a very degraded signal-to-noise ratio. Therefore, the movement velocity of the object to be measured cannot correctly be measured.
With regard to the problems described above, it is generally possible to extract only the AC component by removing the DC component from a signal received by a photodiode 102 and subjected to photoelectric conversion by various BPF's (Bandpass Filters) or the like and by amplifying the signal component in an amplifier circuit so as to obtain a signal of a high signal-to-noise ratio, as shown in FIG. 14. In FIG. 14, the reference numeral 110 denotes an HPF (Highpass Filter), 112 and 113 denote LPF's (Lowpass Filters), and 111 denotes resistors and capacitors for determining the time constants of the LPF's 112 and 113.
However, the AC components of the signal measured by the Doppler velocimeter include not only the Doppler frequency shift of the object to be measured but also high-frequency and low-frequency noises, and this leads to a problem that the AC noises disadvantageously pass through the BPF, making the velocity detection difficult. Among these, a particularly serious problem is the low-frequency noises. In general, the objects of which the velocity is to be detected by an LDV include various objects of powders, fluids, solid surfaces and so on. Since the objects are moving, the reflected light intensity is changed by the variation in the surface reflectance on a solid surface and by the magnitude of the density of the included particles in the cases of powders and fluids. Therefore, a noise having a frequency attributed to the change is generated. As described above, the LDV signal includes the low-frequency noise component ascribed to the variation in the reflected light intensity. The BPF is able to detect the velocity by setting the low-frequency noise component out of the band and setting the Doppler signal inside the band. However, since the frequency of the low-frequency noise component is increased and decreased in accordance with the magnitude of the movement velocity of the object to be measured with the Doppler frequency of the object to be measured. Therefore, the frequency of the low-frequency noise component is increased when the movement velocity is increased and disadvantageously enters the passband of BPF, producing a signal of a low signal-to-noise ratio. Moreover, the problem can be avoided by setting the BPF passband in a sufficiently high frequency region with respect to the movement velocity range of the object to be measured so that the low-frequency noises do not enter the passband. However, the detectable velocity range becomes very narrow, and this significantly limits the practical use range of the LDV.
The present invention has been accomplished in view of the problems and has an object to provide an optical movement information detector capable of detecting the movement velocity of the object to be measured with high accuracy in a wide range of velocity band as well as a movement information detection system, electronic equipment and an encoder employing the detector.
Another object of the present invention is to provide an optical movement information detector capable of calculating displacement information by using detected movement velocity and time information as well as a movement information detection system, electronic equipment and an encoder employing the detector.